The present invention relates to an image forming apparatus such as electro-photographic copier and laser printer and more particularly to a heating mechanism for heating, melting and fixing a toner image onto paper.
A copier employs a so-called belt-type heating mechanism. Features of the belt-type heating mechanism is low power consumption, short warming-up time, oilless, separation characteristic and the like.
The belt-type heating mechanism comprises a fixing roller, a heating roller, a fixing belt (hereinafter referred to as a heat transferring belt) stretched between the fixing roller and the heating roller and a pressurizing roller disposed so as to oppose the fixing roller through the heat transferring belt. The heat transferring belt is heated by the heating roller and fed to the fixing roller. A so-called nip portion is formed between the heat transferring belt and the pressurizing roller and toner of a toner image is melted under a pressure, so that the toner image is fixed on the paper.
In such a conventional belt-type heating mechanism, the heat transferring belt shifts in a width direction during a rotation of the fixing roller/heating roller, so that the heat transferring belt deviates from an image forming range, and consequently, a non-fixed image region may be sometimes generated.
Jpn. Pat. Appln. KOKAI Publication No. 2000-338812(hereinafter referred to as a xe2x80x9cpreceding inventionxe2x80x9d) has proposed installation of a restricting member onto a roller in order to restrict the deviation of the heat transferring belt. However, when in the belt-type heating mechanism of the preceding invention, a side portion of the belt makes a sliding contact with the restricting member, overload is applied to the belt, so that buckling or swelling occurs in the belt.
Thus, in the heating mechanism of the preceding invention, a release agent roller is pressed to the belt so as to coat the belt with the release agent (oil) and apply an additional tension thereto, thereby preventing the belt from being buckled or swollen. Further, in the heating mechanism of the preceding invention, the release agent roller is set up shorter than the width of the fixing belt in order to prevent the release agent from invading to a rear side of the belt.
However, if the belt width is larger than the release agent roller for applying a tension, a tension shortage region in which no sufficient tension is applied to the belt by the release agent roller is generated in a belt side portion. Particularly, in a portion on the side in which the belt shifts in the width direction, this tension shortage region is expanded. If the belt side short of tension makes a sliding contact with the restricting member, a local buckling or swelling (out-of-plane deformation) is generated in the belt and finally, the belt may be broken.
An object of the present invention is to provide a heating mechanism for use in an image forming apparatus capable of fixing a toner image on a paper stably without generating a buckling or swelling (out-of-plane deformation) in the belt.
To achieve the above object, according to an aspect of the present invention, there is provided a heating mechanism for use in an image forming apparatus, comprising: a paper transporting path for carrying a paper having a toner image; a fixing roller disposed in the paper transporting path so that it is rotated; a pressurizing roller disposed so as to oppose the fixing roller across the paper transporting path so that it contacts one side face of a paper carried on the paper transporting path, the pressurizing roller being rotated synchronously with the fixing roller for applying a pressure on the paper in cooperation with the fixing roller; a heating roller provided apart from the fixing roller for applying heat indirectly to the fixing roller; a heat transferring belt which is wound around the heating roller and the fixing roller in order to transmit a rotation driving force from the fixing roller to the heating roller and thermal energy from the heating roller to the fixing roller, makes a contact with the other side face of the paper passing between the fixing roller and the pressurizing roller, and melts toner on the toner image by heat to fix the toner image on the paper; and a pair of first restricting members disposed with a predetermined gap from both end faces of the heating roller to guard both sides of the heat transferring belt on the heating roller thereby restricting a shifting motion of the heat transferring belt in a width direction.
When the belt shifts in the width direction on the heating roller, a side portion of the belt is intercepted by the first restricting member so that a further shift in the width direction of the belt is restricted. Consequently, an excessive deviation of the belt from the heating roller is eliminated, so that a buckling or swelling (out-of-plane deformation) of the side portion of the belt is suppressed. In this case, the belt side is permitted to deviate from a peripheral face of the heating roller if it is slight. The reason is that unless the deviation amount of the belt side from the peripheral face of the heating roller exceeds a predetermined value, the tension applied to the belt side does not drop excessively, and therefore, substantially the belt side portion is not deformed. Therefore, even when the first restricting member is provided apart from an end face of the heating roller, if a gap G1 is set to a predetermined value or less, the buckling or swelling of the belt side portion is never generated. In this case, it is desirable that the gap G1 is not more than 1.5 mm.
The aforementioned first restricting member is desired to be disposed on the side in which the belt is fed from the heating roller. On the side (loose side) in which the belt is fed from the heating roller, the belt tension drops so that the buckling or swelling (out-of-plane deformation) is likely to occur on the belt side. Thus, by disposing the first restricting member on the belt loose side of the heating roller, the buckling or swelling (out-of-plane deformation) of the belt side can be prevented effectively.
Preferably, the heating mechanism in an image forming apparatus further comprises a pair of second restricting members disposed with a predetermined gap from both end faces of the fixing roller to guard both sides of the heat transferring belt. Because the second restricting member restricts a shift motion in the width direction of the belt on the fixing roller, synergistic effect is exerted with the restriction by the first restricting member on the heating roller side. That is, if the second restricting member is provided on the side of the fixing roller, the total length of the first and second restricting members which the belt side makes a sliding contact with is increased and therefore, a local stress concentration on the belt side is relaxed, so that the belt side is not buckled.
Preferably, the second restricting member is disposed on the side (loose side) in which the heat transferring belt is fed from said fixing roller. The belt tension drops on the side (loose side) in which the belt is fed from the fixing roller, so that the buckling or swelling (out-of-plane deformation) is likely to occur on the belt side. Thus, by disposing the second restricting member on the belt loose side of the fixing roller, the buckling or swelling (out-of-plane deformation) on the belt side can be prevented effectively.
Meanwhile, the width LB of the belt needs to be smaller than the length LH of the heating roller and the length of the fixing roller. The reason is that if the width LB of the belt is larger than the roller length, the belt side deviates from the peripheral rolling face peripheral circumferential surface of the heating roller or the fixing roller even when the belt does not shift in the width direction.
Preferably, the heating mechanism further comprises a release agent roller which coats the belt with release agent and is pressed on the entire width LB of the belt so as to apply a tension to the heat transferring belt. In this case, the length Loil of the release agent roller is substantially the same as a clearance between the pair of the first restricting members or a gap G2 between the end face of the release agent roller and the first restricting member is not more than 12 mm.
To apply a tension to the entire belt from the release agent roller, the length Loil of the release agent roller needs to be larger than the width LB of the heat transferring belt.
Preferably, the gap G2 in plan view from an end face of the release agent roller to the first restricting member is not more than 7 mm. If the gap G2 is not more than 7 mm, no deformation occurs in the belt side portion as shown in FIG. 9B. However, if the gap G2 is, for example, 12 mm, the belt side portion is deformed as shown in FIG. 9C, so that the buckling or swelling occurs and finally, the belt is broken. This reason is estimated to be that the tension applied from the release agent roller to the belt acts on a deviated belt side portion from the heating roller. Therefore, by providing with the release agent roller, it is possible to mount the first restricting member apart from the end face of the heating roller.
Meanwhile, it is permissible to employ only a tension roller which applies only a tension to the belt without coating the belt with the release agent, which substitutes the aforementioned release agent roller.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.